Various devices have been proposed for collecting samples of liquid, particularly blood, for testing and analysis. Needle and syringe assemblies are known which rely on either the circulatory pressure of the patient's blood, or a vacuum created by retraction of a plunger, to draw the blood sample into the collection tube. Vacuum tubes have been substituted for the syringe component, most notably in the Vacutainer.RTM. device distributed by Becton Dickinson & Co. Devices employing capillary action to draw puddles of blood into a collection device also have been proposed.
The volume of blood to be collected by the phlebotomist will vary depending upon the ultimate analytical technique and testing equipment. Variable volume syringe assemblies have been proposed in which the plunger is retracted until the free volume in the syringe corresponds to the volume of blood to be withdrawn. However, changes in the desired sample size are difficult to register after blood flow has commenced. The prior art therefore lacks a simple and reliable technique for selectively collecting variable volumes of blood and other liquids.
Various additives may be included in a collection tube prior to introduction of a liquid of interest. For example, it is known to have in place a coating, powder or gel, such as a gel separation barrier, in a blood collection tube to facilitate downline testing or analysis. In certain syringe type devices, an additive may impede flow of blood through the inlet or interfere with venting of the tube interior to the outside.
Various syringe type collection devices have been proposed which use the same opening to collect and dispense the liquid sample. Such devices limit the ability to pour off or pipette samples and provide little or no access to the collected liquid for introducing test specific additives.